At the time this vulnerability was first reported, the computer memory used by a billion instances of the string "lol" would likely exceed that available to the process parsing the XML.
While the original form of the attack was aimed specifically at XML parsers, the term may be applicable to similar subjects as well.
[4] Defenses against this kind of attack include capping the memory allocated in an individual parser if loss of the document is acceptable, or treating entities symbolically and expanding them lazily only when (and to the extent) their content is to be used.
When an XML parser loads this document, it sees that it includes one root element, "lolz", that contains the text "&lol9;".
After all the entity expansions have been processed, this small (< 1 KB) block of XML will actually contain 109 = a billion "lol"s, taking up almost 3 gigabytes of memory.